I imported obj file but it seems it imported as single mesh?
Your .obj file likely doesn't separate objects using the o attribute followed by the object name or g attribute followed by the group name. Either one of those are needed to separate the scene into separate meshes. There seem to be several post in the Autodesk forums regarding the topic of exporting obj as separate meshes, you may find a solution there. Since I don't own a license of 3ds Max, I can't test whether one of them works.

In case 3ds Max doesn't provide a good option to accomplish this, perhaps another file format is better suited, e.g. .fbx, .dae, .ply, .gltf to name a few.